The present invention relates to a water resistant thermoplastic composition, comprising a thermoplastic polymer, an antistatic agent, and a polysiloxane-polyether copolymer to render the composition permanently antistatic.
Polycarbonate based compositions (hereinafter polycarbonate will also be referred to as “PC”) are known for their excellent properties in terms of transparency, heat resistance and mechanical strength. Therefore, they are used in a broad variety of applications, for example lighting applications, such as automotive head lamp lenses, covers and lenses for various illuminating devices, transparent films and sheets, optical disks and optical disk cartridges, various parts used in office equipment and household appliances, and case materials used for storage and transportation, etc.
In particular, in applications where optical properties or aesthetic appearance of plastic parts are significant, the electrostatic nature of PC surfaces in general can cause problems. Accumulation of electrical charges on the polymeric parts causes dust and dirt from the environment to be attracted to the polymeric surfaces. Because PC is an electrical insulator material, irregular surface charge distributions on the surface occur. As a result visually disturbing dust or dirt attraction patterns e.g. fernlike marks arise, especially after prolonged exposure or after electrostatic charging in the final application.
In order to overcome the above problems essentially two ways have been suggested. The first one consists of applying external antistatic additives, for instance by spraying an antistatic coating formulation onto a final part or by dipping the part into the coating. The second method consists of adding an internal antistatic additive directly to the thermoplastic polymer composition. The antistatic additive needs to have a presence at the surface of the final part.
Antistatic additives used in such compositions enable dissipation of electrical surface charges. Most commonly they attract water to the polymeric surface, thereby forming a conductive surface layer. For optical quality PC applications, esters of branched fatty acids and polyhydroxylic compounds and phosphonium sulfonates have been suggested as antistatic additives. Especially the alkylphenylsulfonate phosphonium salts have been appreciated for their antistatic effectiveness.
A drawback of the alkylphenylsulfonate phosphonium salts as antistatic agents is that the required concentration levels for satisfactory antistatic behavior and persistence are high. In practice, they are higher than one would like, thereby other desirable properties are adversely affected. Substituted alkylsulfonate phosphonium salts as disclosed in U.S. Pat. No. 6,194,497 to Willems, particularly perfluoronated alkylsulfonate phosphonium salts, are effective antistatic agents at lower loadings. United States application 2003-65071A to Scholten discloses compositions comprising an antistatic agent and small amounts (0.01–1 parts) of a silicon oil, wherein the silicon oil acts as an antistatic activity enhancer. The enhancing function of the silicon oil is that it improves the permanency of the antistatic characteristic upon heat aging. Nothing is indicated however about the resistance and the permanency of the antistatic character upon exposure to water.
There remains a need in the art for water resistant permanent antistatic compositions, particularly upon exposure to water.